Brazing method using homogeneous low melting point copper based alloys

ABSTRACT

A process of brazing using a copper based low melting point metal alloy composition consisting essentially of about 2.5 to 11 atom percent tin and about 11 to 15 atom percent boron, the balance being essentially copper and incidental impurities. The composition is such that the total of copper and tin ranges from about 85 to 89 atom percent.

This application is a division of application Ser. No. 420,550, filedSept. 20, 1982, now U.S. Pat. No. 4,448,851.

DESCRIPTION BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to copper based metal alloys and moreparticularly to a homogeneous, ductile brazing material useful forbrazing metal articles such as those composed of copper and copperalloys.

2. Description of the Prior Art

Brazing is a process of joining metal parts, often of dissimilarcomposition, to each other. Typically, a filler metal that has a meltingpoint lower than that of the metal parts to be joined together isinterposed between the metal parts to form an assembly. The assembly isthen heated to a temperature sufficient to melt the filler metal. Uponcooling, a strong, leak-tight joint is formed. Filler metals used arecommonly in powder, wire or foil form depending on the type ofapplication. Foil form provides the advantage of preplacing the fillermetal in the joint area, thus permitting brazing of complex shapes withminimum rejection.

The brazing alloys suitable for use with copper and copper alloys,designated AWS BAg are well known compositions. These alloys containsubstantial amounts of the precious metal silver (19 to 86 weightpercent) and hence are expensive. Most of the AWS BAg compositions arefabricated to a foil form through a lengthy sequence of rolling andannealing, thereby incurring substantial processing cost.

Ductile glassy metal alloys have been disclosed in U.S. Pat. No.3,856,513, issued Dec. 24, 1974 to H. S. Chen et al. These alloysinclude compositions having the formula T_(i) X_(j), where T is at leastone transition metal and X is an element selected from the groupconsisting of phosphorus, boron, carbon, aluminum, silicon, tin,germanium, indium, beryllium and antimony, "i" ranges from about 70 toabout 87 atom percent and "j" ranges from about 13 to 30 atom percent.Such materials are conveniently prepared in powder, wire or foil form byrapid quenching friom the melt using processing techniques that are nowwell-known in the art. However, no liquid-quenched glassy metal alloysof the family T_(i) X_(j) described above, containing copper as theprincipal transition metal have been reported. Chen et al. report onlyone copper containing composition (e.g. Pd₇₇.5 Cu₆ Si₁₆.5) in U.S. Pat.No. 3,856,513. H. Suto and H. Ishikawa, Trans. Japan Inst. of Metals, V.17, 1976, p. 596, report fabrication of glassy Cu-Si by vapordeposition.

There remains a need in the art for a homogeneous brazing material forjoining copper and copper alloys that is free of precious metals and canbe produced in foil, powder or wire form.

SUMMARY OF THE INVENTION

The present invention provides a low melting point copper based metalalloy composition. Generally stated the composition consists essentiallyof about 2.5 to 11 atom percent Sn, about 11 to 15 atom percent B,balance being essentially Cu and incidental impurities. The compositionbeing such that the total of Cu and Sn ranges from about 85 to 89 atompercent. Preferably, the metal alloy composition has at least partiallyglassy structure.

In addition, the invention provides a homogeneous, ductile brazing foilhaving a composition consisting essentially of about 2.5 to 11 atompercent Sn, about 11 to 15 atom percent B, balance being essentially Cuand incidental impurities with total of Cu and Sn ranging from about 85to 89 atom percent. Preferably the brazing foil of this invention is atleast partially glassy and consists essentially of about 75 to 78 atompercent copper, about 10 to 11 atom percent Sn and about 11 to 13 atompercent B.

It has been found that the addition of Sn markedly increases thestrength of joints brazed with alloys of this invention. The presence ofthe metalloid component, B, serves to depress the melting point of theCu constituent and provides the alloy with self-fluxing capability.

The homogeneous brazing foil of the invention is fabricated by a processwhich comprises forming a melt of the composition and quenching the melton a rotating quench wheel at a rate of least about 10⁵ ° C./sec.

Further, there is provided in accordance with the invention, an improvedprocess for joining two or more metal parts by brazing. The processcomprises:

(a) interposing a filler metal between the metal parts to form anassembly, the filler metal having a melting temperature less than thatof any of the metal parts;

(b) heating the assembly to at least the melting temperature of thefiller metal; and

(c) cooling the assembly. The improvement comprises employing, as thefiller metal, a homogeneous, copper based foil that has the compositiongiven above.

The filler metal foil is easily fabricable as homogeneous, ductileribbon, which is useful for brazing as cast. Advantageously, the copperbased metal foil can be stamped into complex shapes to provide brazepreforms.

Advantageously, the homogeneous, ductile brazing foil of the inventioncan be placed inside the joint prior to the brazing operation. Use ofthe homogeneous, ductile copper based foil provided by this inventionalso permits brazing to be accomplished by processes such as dip brazingin molten salts, which are not readily accomplished with powder orrod-type fillers.

DETAILED DESCRIPTION OF THE INVENTION

Glassy metal alloys are formed by cooling a melt of the desiredcomposition at a rate of at least about 10⁵ ° C./sec. A variety of rapidquenching techniques, well known to the glassy metal alloy art, areavailable for producing glassy metal powders, wires, ribbon and sheet.Typically, a particular composition is selected, powders or granules ofthe requisite elements in the desired portions are melted andhomogenized, and the molten alloy is rapidly quenched on a chillsurface, such as a rapidly rotating cylinder, or in a suitable fluidmedium, such as water.

Copper based brazing alloys have been fabricated by processes such asthose described above.

In any brazing process, the brazing material must have a melting pointthat will be sufficiently high to provide strength to meet servicerequirements of the metal parts brazed together. However, the meltingpoint must not be so high as to make difficult the brazing operation.Further, the filler material must be compatible, both chemically andmetallurgically, with the materials being brazed. The brazing materialmust be more noble than the metals being brazed to avoid corrosion.Ideally, the brazing material must be in ductile foil form so thatcomplex shapes may be stamped therefrom. Finally, the brazing foilshould be homogeneous, that is, contain no binders or other materialsthat would otherwise form voids or contaminating residues duringbrazing.

In accordance with the invention, a homogeneous, ductile brazingmaterial in foil form is provided. The brazing foils includecompositions ranging from about 2.5 to 11 atom percent Sn, about 11 to15 atom percent B, balance being essentially Cu and incidentalimpurities.

These compositions are compatible with copper and copper-based alloysand are particularly suited for joining these materials.

By homogeneous is meant that the foil, as produced, is of substantiallyuniform composition in all dimensions. By ductile is meant that foil canbe bent to a round radius as small as ten times the foil thicknesswithout fracture.

Examples of brazing alloy compositions within the scope of the inventionare set forth in Table I.

Within the broad range disclosed above, there is a preferred compositionrange that is compatible with and permits brazing of copper and a widerange of copper alloys under a wide range of atmospheric conditions.Such preferred composition range permits copper and copper alloys to bejoined under substantially all brazing conditions. A specially preferredalloy composition of the present invention consists essentially of about77 atom percent Cu, about 11 atom percent Sn and about 12 atom percentB.

Further, in accordance with the invention, an improved process forjoining two or more metal parts is disclosed. The process comprises:

(a) interposing a filler metal between the metal parts to form anassembly, the filler metal having a melting temperature less than thatof any of the metal parts;

(b) heating the assembly to at least the melting temperature of thefiller metal; and

(c) cooling the assembly. The improvement comprises employing, as thefiller metal, at least one homogeneous, copper based foil having acomposition within the ranges given above.

The brazing foils of the invention are prepared from the melt in thesame manner as glassy metal foils. Under these quenching conditions, ametastable, homogeneous, ductile material is obtained. The metastablematerial may be glassy, in which case there is no long range order.X-ray diffraction patterns of glassy metal alloys show only a diffusehalo, similar to that observed for inorganic oxide glasses. Such glassyalloys should be at least 50% glassy to be sufficiently ductile topermit subsequent handling, such as stamping complex shapes from ribbonsof the alloys. Preferably, the glassy metal alloys should be totallyglassy, to attain superior ductility.

The metastable phase may also be a solid solution of the constituentelements. In the case of the alloys of the invention, such metastable,solid solution phases are not ordinarily produced under conventionalprocessing techniques employed in the art of fabricating crystallinealloys. X-ray diffraction patterns of the solid solution alloys show thesharp diffraction peaks characteristic of crystalline alloys, with somebroadening of the peaks due to desired fine-grained size ofcrystallites. Such metastable materials may also be ductile whenproduced under the conditions described above.

The brazing material of the invention is advantageously produced in foil(or ribbon) form, and may be used in brazing applications as cast,whether the material is glassy or a solid solution. Alternatively, foilsof glassy metal alloys may be heat treated to obtain a crystallinephase, preferably fine-grained, in order to promote longer die life whenstamping of complex shapes is contemplated.

Foils as produced by the processing described above typically are about0.0010 to 0.0025 inch (25.4 to 63.5 μm) thick, which is also the desiredspacing between bodies being brazed. Such spacing maximizes the strengthof the braze joint. Thinner foils stacked to form greater thicknessesmay also be employed. Further, no fluxes are required during brazing,and no binders are present in the foil. Thus, formation of voids andcontaminating residues is eliminated. Consequently, the ductile brazingribbons of the invention provide both ease of brazing, by eliminatingthe need for spacers, and minimal post-brazing treatment.

The brazing foils of the invention are also superior to various powderbrazes of the same composition in providing good braze joints. This isprobably due to the ability to apply the brazing foil where the braze isrequired, rather than depending on capillarity to transport braze fillermetal from the edge of surfaces to be brazed.

EXAMPLE 1

Ribbons about 2.5 to 6.5 mm (about 0.10 to 0.25 inch) wide and about 25to 60 m (about 0.0010 to 0.0025 inch) thick were formed by squirting amelt of the particular composition by overpressure of argon onto arapidly rotating copper chill wheel (surface speed about 3000 to 6000ft/min). Metastable, homogeneous alloy ribbons having at least partiallyglassy atomic structure were produced and the compositions of theribbons are set forth in Table I.

                  TABLE 1                                                         ______________________________________                                        Sample No.          Cu         Sn   B                                         ______________________________________                                        1          atom %   86.5        2.5 11.0                                                 wt. %    93.0        5.0  2.0                                      2          atom %   77.0       11.0 12.0                                                 wt. %    77.0       21.0  2.0                                      3          atom %   74.0       11.0 15.0                                                 wt. %    76.0       21.0  3.0                                      ______________________________________                                    

EXAMPLE 2

The liquidus and solidus temperatures, T_(L) and T_(S) of the selectedcomposition (atom %) Cu₇₇ Sn₁₁ B₁₂ were determined by DifferentialThermal Analysis (DTA) techniques. The temperatures are set forth inTable II.

                  TABLE 2                                                         ______________________________________                                        Sample No.                                                                              Composition  T.sub.L °C. (°F.)                                                         T.sub.S °C. (°F.)              ______________________________________                                        2    atom %   Cu.sub.77 Sn.sub.11 B.sub.12                                                               898 (1648)                                                                            784 (1443)                                 ______________________________________                                    

EXAMPLE 3

Lap shear test specimens were prepared according to the AWS C 3.2"Standard Method for Evaluating the Strength of Brazed Joints." Coppersheet, 3.175 mm (0.125") thick was used as the base metal. Ribbons ofthe selected composition (atom %) Cu₇₇ Sn₁₁ B₁₂ having dimensions ofabout 25.4 μm to 38.1 μm (0.001"-0.0015") thick and about 6.35 mm(0.25") wide were used as the filler metal. Brazed joints were of thelap type with the lap dimension carefully controlled to 6.35 mm (0.25")and 12.7 mm (0.5"). Specimens were then degreased in acetone and rinsedwith alcohol. The mating surfaces of the blanks were fluxed using boricacid. Lap joints containing the selected brazing ribbon of the inventionwas then assembled by laying ribbons side by side to cover the entirelength of the lap joint. Specimens were then clamped and torch brazedusing oxyacetylene flame with 8 psi oxygen and 8 psi acetylene pressure.Brazed specimens were then air cooled to room temperature and the fluxresidue was removed by wire brushing.

For comparative purposes identical joints were prepared using 25.4 μm(0.001") thick BCuP-5 foil and 0.157 cm (0.064") dia BAg-1 and BAg-2rod. The nominal compositions and brazing temperature ranges of thesefiller metals are given in Table IIIA and IIIB, respectively.

                  TABLE IIIA                                                      ______________________________________                                        Alloy            Ag      Cu   P      Zn   Cd                                  ______________________________________                                        BCuP-5  atom %   8.92    80.73                                                                              10.35  --   --                                          wt. %    15      80   5      --   --                                  BAg-1   atom %   37.53   21.24                                                                              --     22.02                                                                              19.21                                       wt. %    45      15   --     16   24                                  BAg-2   atom %   26.71   33.67                                                                              --     26.44                                                                              13.18                                       wt. %    35      26   --     21   18                                  ______________________________________                                    

                  TABLE IIIB                                                      ______________________________________                                        Alloy       Temp. °C. (°F.)                                     ______________________________________                                        BCuP-5      704-816 (1300-1500)                                               BAg-1       618-760 (1145-1400)                                               BAg-2       635-760 (1175-1400)                                               ______________________________________                                    

When the applied filler metal was in rod form (BAg-1 and BAg-2 alloys),a clearance of 38.1 μm (0.0015") was kept between the mating surfaces ofthe blank by placing stainless steel spacers at the two edges. Theassembly was then heated to the brazing temperature range of thesealloys and the filler metal was applied to one side only. The moltenfiller metal was then drawn by capillary action and covered the entiremating surfaces. Mechanical properties of brazed joints having anoverlap of 12.7 mm (0.5 inch) are listed in Table IVA, while mechanicalproperties of brazed joints having an overlap of 6.35 mm (0.25 inch) areset forth in Table IVB.

                  TABLE IVA                                                       ______________________________________                                               Shear Strength                                                                              Tensile Strength                                                                           Area of                                     Alloy  Mpa (psi)     Mpa (psi)    Failure                                     ______________________________________                                        BCuP-5 44 (6,320)    174 (25,280) Joint                                       BAg-1  41 (6,660)    184 (26,640) Joint                                       BAg-2  43 (6,240)    172 (24,960) Joint                                       Sample 2                                                                             46 (6,610)    182 (26,440) Base                                                                          Metal                                       ______________________________________                                    

                  TABLE IVB                                                       ______________________________________                                               Shear Strength                                                                              Tensile Strength                                                                           Area of                                     Alloy  Mpa (psi)     Mpa (psi)    Failure                                     ______________________________________                                        BCuP-5 93 (13,440)   185 (26,880) Joint                                       BAg-1  72 (10,440)   144 (20,880) Joint                                       BAg-2  62 ( 9,040)   125 (18,080) Joint                                       Sample 2                                                                             94 (13,660)   188 (27,320) Base                                                                          Metal                                       ______________________________________                                    

At overlaps of both 12.7 mm (0.5 inch) and 6.35 mm (0.25 inch), theselected alloy of the present invention having the composition (atompercent) Cu₇₇ Sn₁₁ B₁₂ failed in the base metal, indicating the strengthof the brazed joint exceeded that of the base metal. On the contrary,identical brazements made with the silver containing alloys BCuP-5,BAg-1 and BAg-2 failed in the brazed joints at overlaps of 12.7 mm (0.5inch) and 6.35 mm (0.25 inch). Therefore, the selected alloy of thepresent invention having the composition (atom percent) Cu₇₇ Sn₁₁ B₁₂produced stronger joint compared to the silver containing alloys BCuP-5,BAg-1 and BAg-2.

Having thus described the invention in rather full detail, it will beunderstood that such detail need not be strictly adhered to but thatvarious changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the present invention asdefined by the subjoined claims.

What is claimed is:
 1. A process for joining together two or more metalparts which comprises:(a) interposing a filler metal between the metalparts to form an assembly, the filler metal having a melting point lessthan that of any of the parts; (b) heating the assembly to at least themelting temperature of the filler metal; and (c) cooling the assembly;wherein the improvement comprises employing, as the filler metal, ahomogeneous copper based foil having a composition consistingessentially of about 2.5 to 11 atom percent tin, about 11 to 15 atompercent boron, the balance being copper and incidental impurities andthe total of copper and tin ranging from about 85 to 89 atom percent. 2.The process of claim 1 in which the filler metal foil has at least about50% glassy structure.
 3. The process of claim 1 in which the fillermetal has at least partially glassy structure.
 4. The process of claim 1in which the ductile filler metal foil has a composition consistingessentially of about 10 to 11 atom percent tin, about 11 to 13 atompercent boron and about 75-78 atom percent copper.
 5. The process ofclaim 1 in which the ductile filler metal foil has a compositionconsisting essentially of about 77 atom percent copper, about 11 atompercent tin and about 12 atom percent boron.